Wireless G is 54mbps (max) so you must be getting a poor signal. So the router is in the attic....is that right above you? Or above a different room? I can use my laptop anywhere in my house (about 1500 sq ft) and I don't connect at anything less than 48 mbps (wirelessG as well). My router is located in the center of the house so the farthest away I get is maybe 50 ft with 3 walls to go through.

Are there any metal ducts or other metal things in your attic that may be interfering? If I stand on the opposite side of the furnace my signal strength plummets.

And you were only getting 8 mbps before you put it in the attic? If so....that seems okay. You're loosing a bit of performance because it's now in the attic and has to go through the ceiling.